The frequently changing IP address and password change may cause this issue.

Please check if you:1. Use VPN or proxy, or use AirDroid on different computer/network (frequently changing IP address)2. Deleted cookies (saved password will be deleted)3. Signed out from web.airdroid.com4. Changed password of AirDroid5. Signed in to web.airdroid.com by scanning QR Code, while there's not AirDroid account signed in on that device